Method of establishing an electrical charge on a conductive interlayer unconnected to a potential source



Nov. 24, 1970 Vm VOLTS W. YELLIN E METHOD OF ESTABLISHING AN ELECTRICAL CHARGE ON A c0 INTERLAYER UNCONNEC'IED TO A POTENTIAL SOURCE Filed Nov. 21. .1966

NDUCTTVI'I 3 Sheets$heet 1 BREAKDOWN POTENTIAL IN DRY AIR I vs. INTERLAYER THICKNESS, LAND SPACING X- IN V EN TORS JOHN W. WEIGL ATTORNEYS Nov. 24; .1970 w. YELLIN ETAL 3,543,023

METHOD OF ESTABLISHING AN ELECTRICAL CHARGE ON A CONDUCTIVE INTERLAYER UNCONNECTED TO A POTENTIAL SOURCE Filed Nov. 21, 1966 3 Sheets-Sheet 2 'L M '0 I L l4 13 E 1 7 v i j Qua L FIGZ INVENTORS JOHN W. WEIGL WILBUR ELLIN W W ATTORNEYS Nov. 24, 1970 w, EIAL 3,543,023

METHOD OF ESTABLISHING AN ELECTRICAL CHARGE ON A CONDUCTIVE INTERLAYER UNCONNECTED TO A POTENTIAL SOURCE Filed Nov. 21, 1966 5 Sheets-Sheet 3 FIG. 7

INVENTORS JOHN W. W

B WILBUR YE k4, w W ATTORNEYS United States Patent U.S. Cl. 250-495 5 Claims ABSTRACT OF THE DISCLOSURE A method and its associated apparatus, for electrostatic electrophotography whereby a charge is imparted on a conductive interlayer of electrographic material without direct mechanical contacting.

This application is a continuation-in-part of our application Ser. No. 136,561, filed Sept. 7, 1961, which was expressly abandoned in view of the filling of this application.

This invention relates to improvements in electrophotographic recording and, more particularly, to a method and means for establishing a charge of static electricity on a direct-image electrophotographic medium usually prior to its image-wise exposure to light.

.In the usual form of electrostatic recording or xerography, an offset system is employed wherein the final recording is obtained on a sheet, usually of paper, by contact of the paper with an image or a latent electrostatic image on an offset recording medium. The standard recording medium consists of an electrically conductive base, usually of metal, over which is coated an insulating resinous binder in which is dispersed a photoconductive material. A static charge of electricity is readily applied to such a medium by merely connecting the conductive base to an electrode, whereby, in the absence of actinic radiation, a certain electrical charge is imparted to the conductive base and the bottom of the photoconductive layer while an opposite charge appears on the upper surface of the photoconductive layer which, in the dark, is electrically insulating. The charge on certain portions of the upper surface is changed in an imagewise fashion when the recording medium is exposed to actinic radiation, producing an electrostatic latent image, and an electrically charged toner powder or developing fiuid applied to the medium adheres differentially to the latent image. The developer or toner is then usually offset to the sheet on which the image is to be recorded.

In direct-image recording, a fibrous or resinous base is used, which generally is electrically insulating, as distinguished from the electrically conductive base in the offset process. However, it has been found that a stronger charge, resulting in a denser final image, may be obtained in direct-image recording by using an improved form of recording medium comprising an insulating resinous or fibrous base over which is coated a conductive interlayer, the latter supporting a photoconductive coating which may consist, for example, of zinc oxide and a binder. The interlayer type of material also has certain advantages in economy, appearance, and ease of handling. At example of this type of recording material is found in French Pat. 1,220,481.

In utilizing this interlayer type of recording medium, the interlayer is given a charge of static electricity and the art has applied such charge by a physical connection of this interlayer to one terminal of the charging potential source which is generally the terminal at ground or reference potential, referred to as ground. However, a problem with such photographic elements has been the assuring of mechanical contact, during charging of the interlayer with the charging electrode.

Accordingly, it is a primary object of this invention to provide a method and means for establishing a charge on the conductive interlayer of electrographic materials without the use of direct mechanical contacting. It is a further object of the invention to provide means for obtaining improved electrostatic reproduction by more accurate control of the conductive interlayer potential. It is a particular feature of the invention that the apparatus employed is simple and easily adaptable to any existing electrophotographic equipment.

In this invention a charge is applied to the conductive interlayer without any mechanical connection between the interlayer and an electrode, by causing a corona discharge. In this method, a source of potential is connected to an electrode positioned adjacent an outer surface of the recording medium, that is, to the insulating base and/ or to the normally (dark) insulating photoconductive layer. By establishing a sufficient potential difference between the conductive interlayer and the electrode, an effective electrical contact may be established between the conductive interlayer and the electrode across narrow gaps which include air and/ or electrically-insulating solid material without a need for mechanical contact with the interlayer or even, often, with the recording medium at all. This contact is established by ionic gas conduction caused by corona discharge. This discharge will occur in air between two differently charged conductive bodies and is enhanced if at least one conductor has sharp edges or points.

In accordance with the invention, the conductive interlayer may advantageously be utilized as the sharp-edged member. The reason for this is that, in practice, the thickness of the interlayer usually does not exceed about 0.002 inch and is generally about 0.0004- inch thick. Such minute thickness represents, in fact, a very sharp-edged electrode.

This invention will be more readily understood by reference to the accompanying drawings, which are to be considered illustrative only and not limiting and in which:

FIG. 1 is a graph of breakdown potential in air as a function of interlayer distance to an electrode and interlayer thickness illustrating the operating conditions of the principles employed in this invention;

FIG. 2 is an enlarged schematic sectional view of a preferred embodiment of the charging means;

FIG. 3 is a schematic view in perspective of the application of the preferred embodiment in the charging step of an electrostatic recording apparatus;

FIG. 4 is a similar view illustrating an alternate embodiment of the invention;

'FIG. 5 is an enlarged sectional view illustrating a modified charging arrangement;

FIG. 6 is an enlarged sectional view of another embodiment;

FIG. 7 is a schematic view, partially, in section, of the application of the embodiment of FIG. 5 in a charging step of an electrostatic recording apparatus.

As seen in the graph in FIG. 1, if an electrode at potential V is placed within a distance x of the edge of a conductive interlayer of thickness t which is initially at ground potential, a maximum field intensity E is set up at the edge. The maximum field intensity is represented by For a'ir, breakdown occurs at an intensity of about 30 kv./cm. Employing this figure for E and solving for the unknown V in the equation, the graph in FIG. 1 was plotted using different values for the interlayer thickness 3 t and the spacing x expressed in centimeters. By providing conditions where corona discharge will take place, the conductive interlayer will acquire a potential substantially equal to that of the electrode. As the humidity of the air rises, the voltage required to reach this condition of corona discharge drops.

In FIG. 2 is shown a greatly enlarged schematic view of a preferred embodiment of the invention. The recording medium 10 comprises an insulating base 12, a conductive interlayer 13, and a photoconductive or other insulating member 14. An electrode 16 placed under the base 12 is connected to the return terminal or ground potential side of a charging voltage source 17. The charging electrodes 18 are above the photoconductive member or layer 14.

The dimensions t (thickness) of the conductive interlayer and x (the spacing between the electrode and the conductive interlayer) are plotted as a function of breakdown potential in dry air in FIG. 1. As mentioned before, the conductive interlayer is usually not greater in thickness than about 0.002 inch, and usually is less than about 0.0004 inch. Moreover, the interlayer usually has even thinner conductive fibers and protrusions along its edges. Thus electrical breakdown in the form of a corona discharge between the electrodes 16 and the interlayer 13 forms easily and effectively grounds the latter without there being any direct mechanical contact between these two elements. 7

FIG. 3 shows the embodiment illustrated in FIG. 2 applied to the charging section of an electrographic apparatus. The electrode 16 takes the form of a bar positioned in a space between a pair of endless transport belts 15'. So as to be substantially parallel to the direction of travel of the recording medium and it is also spaced from the unders-urface of the insulating base 12. As illustrated by way of example, the electrode 16', instead of returning to ground, is connected to a voltage source 17' which is distinct from the charging potential source 17. Both of these voltage sources have a common return to ground. The purpose of the source 17' is to apply a control potential to the electrode 1 6 which, under certain conditions,

is particularly advantageous, e.g., in a reversal development process described in US. Pat. 2,817,598.

FIG. 4 shows a modified arrangement 'of electrode placement. The recording medium 10 is being carried by transport means in the form of an insulating endless belt 15 past a high voltage corona discharge device 18 comprising a set' of tungsten wires 19 mounted above the surface of the photoconductive layer 14. In this arrangement, the electrode 16 takes the form of a fine wire which is imbedded in the belts 15 so that it does not touch the surface of the insulating base 12. Roller contact 20 serves to connect the electrode 16 to the ground potential side of the source 17.

An alternate method of electrically connecting a conductive interlayer isillustrated in FIG. where a plurality of sharply pointed metallic wires in the form of a brush are mounted above the surface of the photoconductive layer 14 which they may freely touch. The recording medium is carried asbefore over an insulating belt 15. The fine wires 29 are mounted close to the lateral edge of the recording medium so as not to'touch such areas on which essential parts of the image will appear. Effective contact is made to the interlayer 13 through the photoconductive layer 14 by local dielectric breakdown, as shown schematically by dotted lines 27. In order to be elfective, the radius of curvature of the sharpest projection from the wire in contact with the insulating layer should be about 0.005 inch.

FIG. 6 shows a modified arrangement for a return circuit contact. Fine wires 46 are imbedded in the edges of insulating transport belts 45 so that these Wires are in contact with the photoconductive layer 14 of the recording medium 10. The fine wires may be replaced by thin strands of semi-conductive thread, or by the wire mesh belts, open-mesh belts of semi-conductive thread, or the like.

Another arrangement, shown in FIG. 7 of the drawings, shows the invention incorporated in a charging section of an electrophotographie recording device. The recording medium 10 is mounted on an insulating drum 30. Insulating side belts 31 carry the recording medium past a fine wire brush 32 consisting preferably of three to fifty wires 33 of stainless steel mounted in a metal tube 35 so as to engage and brush 'over' the free surface of the photoconductive layer 14. Similarly, as disclosed in FIGS. 2 and 4, the conductive interlayer 13 is maintained at ground potential as the recording medium passes beneath the corona discharge unit 18. An insulating shield 34 is inserted between the discharge unit 18 and the grounded brush 32 to prevent sparking between these elements. Eifectivecontact is made between the wire ends 33 and the conductive interlayer 13 without necessitating actual physical contact between these two elements. Analogous devices may be used in the illuminating and developing sections of the apparatus.

It can thus readily be seen that this invention provides an electrophotographic method and apparatus for use with a recording medium which has a conductive interlayer positioned between outer layers of an insulating base and normally insulating, photoconductive, material, respectively. In the invention, an electric charge, which may be different from ground potential, is placed on the conductive interlayer, thereby providing a charge on the outer surface of the photoconductive layer suitable for exposure to produce an electrostatic latent image which may subsequently be developed.

- What is claimed is:

1. In electrostatic electrophotography, the method of effectively establishing an electrical charge on a conductive interlayer positioned between an insulating base and a photoconductive outer layer of a recording medium, said interlayer being further physically unconnected to any potential source, which comprises positioning an electrode adjacent an outside surface of said recording medium without physical contact with said interlayer and connecting said electrode to a potential source of sulficient strength to cause corona discharge between the electrode and said conductive interlayer by which said conductive interlayer may be brought to a desired potential.

2. A method according to claim 1 in which the said electrode contacts the exposed surface'of said photoconductive member.

3. A method according to claim 1 in which the interlayer is charged to a potential other than ground potential.

4. A method according to claim 1 in which the interlayer is no greater than about 0.002 inch in thickness.

5. A method according to claim 1 in which the interlayer is less than about 0.0004 inch in thickness.

References Cited UNITED STATES PATENTS 2,774,921 12/1956 Walkup 317262 2,825,814 3/1958 Walkup 25049.5

FOREIGN PATENTS 203,907 8/1955 Australia.

WILLIAM F. LINDQUIST, Primary Examiner US. 01. X.R. 

